Folded wave signal transmission line



March 28, 1950 WHEELER 2,502,359

FOLDED WAVE SIGNAL TRANSMISSION LINE Filed Jan. 28, 1944 26 3 Q II I4 INVENTOR E HAROLD A. WHEELER 2 Sheets-Sheet 1 March 28, 1950 H. A. WHEELER FOLDED WAVE SIGNAL TRANSMISSION LINE 2 Sheets-Sheet 2 Filed Jan. 28, 1944 Patented Mar. 28, 1950 FOLDED WAVE SIGNAL TRANSMISSION LINE Harold A. Wheeler, Great Neck, N. Y., assignor, by mesne assignments, to Hazeltine Research, Inc., Chicago, 111., a corporation of Illinois Application January 28, 1944, Serial No. 520,016

Claims.

The present invention relates to wave-signal transmission lines and particularly to such lines of the type in which the line is effectively doubled back upon itself lengthwise to form one or more U-shaped loops, thus to minimize the physical size of the line for a given electrical length. While not limited thereto, the invention is particularly suited for use in wave-signal transmission lines of the type last mentioned having means for adjusting their electrical length and will be described in that connection.

It is frequently desirable that a wave-signal transmission line of relatively long electrical length be relatively short and compact. This may be accomplished by doubling each conductor of the line back upon itself lengthwise. For the coaxial type of line, the outer shield conductor is not usually doubled back but rather is simply enlarged to accommodate the doubledback inner conductor. Unless shielding is provided for each such doubled-back conductor along the entire length of its doubled-back section, however, adjacent portions of the conductor where it has been doubled back are electrically and magnetically coupled so that the effective electrical length 01' the line is materially reduced below the value provided by a linear line of the same physical length. This decrease in its effective electrical length becomes more severe as a conductor of the line is doubled back more tightly and in the extreme may defeat the very purpose of the construction.

It has heretofore been proposed that conductive shield members be inserted between the doubled-back portions of the transmission-line conductors to minimize the electric and mag- 'netic coupling between such portions, thus to avoid the decrease of electrical length of the line which otherwise occurs. While shield members of this nature may in general provide the desired result, they materially increase the cost and complexity of a transmission line of this type and may undesirably increase its weight. In such transmission lines which include means for adjusting the physical, and thus the electrical, length of the line, the use of these shield members is quite undesirable since they require the provision of stationary and movable shield sections with intervening sliding contacts or joints which further increase the cost and complexity of the line and the likelihood of impaired operation due to faulty operation of the sliding contacts or joints.

It is an object of the present invention, therefore, to provide a wave-signal transmission line of the type having doubled-back conductors and one which avoids one or more of the abovementioned limitations and disadvantages of prior transmission lines of this type.

It is an additional object of the invention to provide a wave-signal transmission line of adjustable length having a materially simplified and improved construction.

It is a further object of the invention to provide a new and improved wave-signal transmission line of adjustable length and of the type described in which the electromagnetic coupling between adjacent sections of the line conductors is minimized without the need of intervening shield members or sliding contacts or joints.

In accordance with the invention, a wavesignal transmission line comprises an electrically continuous conductor of U-shaped configuration having the legs thereof exposed to one another over a substantial portion of their lengths, and means for adjusting the length of the legs of the conductor. Means is included for providing on one side of the conductor a neutral surface approximately parallel to the median plane of the conductor. The line also includes conductive shield means supported in spaced relation to the other side of the conductor to provide on such other side a neutral surface approximately parallel to the median plane of the conductor, at least one of the neutralsurface-providing means cooperating With the conductor to provide a folded transmission line and the spacing between the legs of the conductor being greater than the spacing between the aforementioned neutral surfaces to reduce the electric and magnetic coupling between the legs, thereby to reduce the tendency of such coupling to shorten the effective electrical length of the folded transmission line.

For a better understanding of the present invention, together with other and further objects thereof, reference is bad to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Referring now to the drawings, Fig. 1 illustrates a wave-signal transmission line of adjustable length embodying the present invention; Fig. 2 represents a cross-sectional view of a shielded transmission line essentially of the coaxial type embodying the Fig. 1 construction; Fig. 3 represents a shielded two-wire or balanced-pair transmission line embodying the Fig. 1 construction; Fig. 4 illustrates a wavesignal transmission line of adjustable length embodying a modified form of the invention; and Figs. 5, 6, 7 and 8 represent cross-sectional it views showing details of construction of the Fig. i arrangement.

Referring now more particularly to Fig. 1 of the drawings, there is illustrated a wave-signal transmission line of adjustable length embodying the present invention in a particular form. This line comprises a continuous U-shaped conductor ill having telescoping leg sections. In

particular, the conductor comprises two linear fixed leg sections ii, I? exposed to one another over a substantial portion of their lengths and supported in parallel relation and a U-shaped section it having leg portions M, i5 adjustably supported in telescopic relation with the respective fixed leg sections ll, 32. The transmission line includes means for adjustin the length of the legs of the conductor M This means includes means for moving the U-shaped section it and means for maintaining substantially constant the characteristic impedance of the line with adjustments of its electrical length comprising a movable rectangular diaphragm Ill. The latter is connected to the U-shaped section by insulators ll, l3 and is arranged to fit closely and move longitudinally within a conductive rectangular housing is which is spaced from and encloses the conductor ill. The diaphragm has spring contact fingers 28 arranged around its periphery to the inner walls of the housing l9 and is adjusted longitudinally of the housing by means of a rod M which extends through an aperture 22 provided in an end wall of the housing. In an alternative construction, the fingers 2!; do not engage the housing is but simply provide capacitive coupling between the diaphragm l6 and the housing. The two linear fixed leg sections la, ii are connected to the inner con-- tacts of individual coaxial jacks 24 which include externally threaded outer contact members extending through individual apertures in an end wall of the housing I!) and electrically and mechanically secured to the latter by lock nuts 26, 26 threaded over the members 25. The contacts are maintained in coaxial relation. with the inner contacts 23 by the provision of insulating bushings ill, 27.

Fig. 2 represents a crcss sectional view taken along the plane 2-4 of Fig. 1 and illustrates a single-conductor shielded transmission line embodying the Fig. 1 construction. Fig. 3 represents a similar cross-sectional view of a twoconductor or balanced shielded transmission line embodying the Fig. 1 construction.

Where the transmission line is a singleconductor shielded line represented by 2, the housing is provides on opposite sides of the conductor is two neutral surfaces approximately parallel to the median plane of the conductor. a neutral surface as used herein and in the appended claims is meant one which provides equi-potential surface for an electric field and one which is not substantially permeated by the lines of force of a magnetic field. Where the transmission line is of the twoconductor or balanced type, as presented by Fig. 3, the two conductors ill, ill thereof are conventionally included in the same electrical circuit so that the currents in the conductos at corresponding points along the line are of the same magnitude but opposltc phase. Hence, in this case, each of the conductors ill provides for the other conductor in a region intermediate the conductors a neutral surface approximately 4. parallel to the median planes of the conductors. In this case, also, the conductive housing l9 provides for each of the conductors IE1, lb and on the outer side thereof a neutral surface as in the Fig. 2 type of transmission line.

The transmission line here illustrated is of the type in which the shielded conductor, or conductors, is folded lengthwise upon itself. Unless prevented, the electrical and magnetic fields produced by each leg of the conductor l0 react on the other leg, producing coupling between the legs and reducing the effective electrical length of the line. Such coupling is reduced in accordance with the present invention by making the spacing between the legs ll, [4 and l2, l5 greater than the spacing between the neutral surfaces which are provided on opposite sides of the conductor ill in the manner previously explained. Where the spacing between the legs is several times the diameter of either leg, the spacing s may be measured between their axes, as indicated in Figs. 2 and 3. However, if the diameters of the legs large in relation to their spacing, then the spacing S must be measured as the distance between adjacent points on their exterior surfaces. If, now, the spacing between the neutral surfaces is D, as indicated in 2 and 3, a ratio of the spacings S to D equal to two reduces the electric and magnetic coupling to a value so small as to be quite satisfactory for most applications. A ratio of the spacings S to D of the order of unity substantially reduces the electric and magnetic coupling between the le s and provides approximately the smallest ratio which is useful in practice.

It is well known that the ratio of the spacing D/Z of the interior wall surfaces of the housing I9 adjacent to the conductor ll], along the length of the latter, to the diameter d or the conductor establishes the desired characteristic impedance of the line. The provision in the present transmission-line construction of the diaphragm it, which moves with the Ll-shaped section 13, ensures that the spacing of the U-shaped section to the three adjacent Wall surfaces of the housing remains constant with adjustments of the length of the line effected by movement of the U-shapcd section and, thus, that the characteristic impedance of the line also remains substantially constant.

Considering now the operation of the transmission line just described, and referred particularly to the single-conductor type of line repre sented by Fig. 2, the metallic housing l9 provides, as previously mentioned, the neutral surfaces on each side of the conductor I0 required to prevent coupling between the legs of the conductor. In this connection, it may be mentioned that the theory oi images is useful in understanding the manner in which the electric and magnetic coupling between the legs is reduced by the neutral surfaces. In accordance with this theory, an image of the conductor 10 is formed on the opposite side of the neutral surface from the conducto'r ID, the conductor and its image being equidistant from the neutral surface. The electric and magnetic fields of the image conductor are formed by image currents of equal amplitude but opposite phase to that of the conductor HI. Hence, either leg of the conductor It is affected not only by the field of the opposite leg but also by that of the opposite leg of the image conductor, so that the fields of the opposite real and image legs would cancel each other at the since the image leg, by the nature of its forma-.

tion, must be at a greater distance from the given leg than is the real leg. This inequality of spacings from the given leg becomes smaller as the spacing between the legs of the conductor ID increases with the result that, for a single neutral surface alone, the electric and magnetic coupling between the legs of the conductor l decreases approximately as the second power of their spacing.

Now where neutral surfaces are provided on opposite sides of the conductor ID, as in the arrangement of Figs. 1 and 2, image conductors are formed not only by the conductor l0 but also by the image conductors and by the images of the image conductors. Either leg of the conductor Ill consequently is afiected by the field of the opposite leg and by those of several image conductors. For this reason, the coupling between the legs of the conductor 10 is reduced, for any given spacing D between the neutral surfaces, approximately exponentially with the spacing S between the legs of the conductor. While the neutral surfaces need not be equidistant from the conductor it, it will be apparent that maximum reduction of coupling is effected by equal spacings since this ensures that the image conductors have optimum spacings to each leg of the conductor m.

The metallic housing l9 not only provides such neutral surfaces, but additionally shields the conductor H! from any external radiant-energy field and restricts the radiation field of the conductor ill to the space enclosed by the housing l9. Additionally, the housing 19 forms one conductor of the transmission line, thus to provide a line in the nature of a coaxial transmission line. The electrical length of the transmission line is adjusted to a desired value by manipulation of the rod 2| to move the U-shaped section l3 of the conductor Ill, thus to adjust the physical and electrical length of the conductor. In a two-conductor or balanced transmission line, of the type represented by Fig. 3, the conductive housing l9 provides one neutral surface for each of the conductors I ll, Ill, while at the same time shielding the conductors from external radiant-energy fields and restricting the radiation fields of the conductors to the space within the housing. As previously men- I tioned, the other neutral surface for each conductor ii], i9 is provided between the conductors by the currents of equal amplitude but opposite phase which are carried by the conductors while operating in normal fashion as a balanced trans- A mission line.

Fig. 4 illustrates a wave-signal transmission line of adjustable length embodyin a modified form of the invention. This line is essentially similar to that of Fig. 1, similar elements being designated by similar reference numerals and analogous elements by similar reference numerals primed, except that the maximum physical length of the line for a given value of maximum electrical length is reduced by the provision of dielectric material 28 which has a dielectric constant greater than unity and which substantially fills the space between the U-shaped section l3 of the conductor ill and the interior wall surfaces of the housing l9. Figs. 5-8, inclusive, represent cross-sectional views of the Fig. 4 construction along cross-sectional planes indicated in Fig. 4 as 55, 6-6, 1-1, and 88, respectively, and represent constructional details of this modified form of transmission line.

sections II and I2.

Since the dielectric material 28 increases the capacitance of the U-shaped section l3 per unit of its length, it is necessary correspondingly to increase the inductance per unit length of this section in order that the characteristic impedance of the line shall remain uniform along its entire length. This is accomplished by constructing the U-shaped section 13 of flat strip material of relatively small cross-sectional area, as will be evident from any of Figs. 5-8, inclusive. In this respect, the ratio of the inductance to the capacitance per unit length of the U-shaped section I3 is made equal to that of the two linear fixed leg The end of each leg section II and I2 is pinched down to engage the fiat sides of the U-shaped section l3, as indicated in Fig. 8. The dielectric material is provided with a rectangular slot 29 to permit longitudinal movement of the U-shaped section l3, the adjusting rod 2|, and an insulating block 36 which surrounds the U-shaped section l3 and mechanically connects it to the adjustin rod 2|, as indicated in Fig. '7.

The operation of this modified form of the invention is essentially similar to that previously described, the U-shaped section it being moved by means of the adjusting rod 2| to adjust the electrical length of the transmission line to a desired value. In this connection, the dielectric material 28 materially decreases the velocity of wave-signal propagation along the line. Thus, the effective electrical length of that portion of the U-shaped section 13 which extends into the dielectric material 28 is materially longer than would be the case were the U-shaped section to be surrounded only by air. A given change of electrical length of the line is consequently effected by a much smaller movement of the U- shaped section l3 than in the case of the Fig. 1 construction. As in the Fig. 1 arrangement, the spacings between the legs ll, 14' and i2, E5 of the U-shaped conductor i0 is greater than the spacing D between the neutral surfaces provided on opposite sides of the conductor by the housing l9, thereby to reduce the electrical magnetic coupling between the legs.

The capacitance between the sides of the U- shaped member [3 and the housing i9 is so large in relation to the capacitance between the edge of the base portion of the U-shaped section and the end of the housing that the latter capacitance is entirely negligible throughout the range of adjustment of the transmission line. There is consequently no need to employ a movable diaphragm supported in spaced relation to the U-shaped member I3, as in the Fig. l construction, for purposes of maintaining the characteristic impedance of the line constant with adjustments of its electrical length. The dielectric material 28 thus comprises, in the present modification, means for maintaining substantially constant the characteristic impedance of the line with adjustments of its electrical length.

The cross sectional area of the material of the U-shaped section l3 may be increased, thus to lend greater rigidity to this section, if a uniformly distributed suspension of finely divided magnetic material is added to the dielectric material 28. This may be provided by a colloidal suspension of iron particles in a liquid dielectric material, or by a uniform mixture of magnetic particles in a solid dielectric material. For example, the dielectric material may comprise titanium dioxide and the magnetic material may be thoroughly mixed with the titanium dioxide whilethe latter is in a plastic state and prior i'to molding and baking the mixture to a desired configuration.

While the transmission llneillustrated in Figs. l-d, inclusive, includes only aslngl'e conductor enclosed within a conductive shield, it will be apparent that a similar construction ill-21y be utilined in a two-conductor balanced transmission line of the type represented by Fig. 3, two transmission lines of the Fig. 4 type siinply'being'en closed withinla conductive housing with the U- shaped sections l3 thereof mechanically connected for simultaneous movement by a single adjusting rod 2|.

Additional-1y, while the conductive housing i=9 7 been illustrated as having 'a rectangular cross section, it will be apparentthat the housing may also have other cross-sectional configurations, for example an elliptical cross section.

From the above description of theinvention, it will be apparent that a wave signal transmission line ei'nbodying the invention involves e. materially simplified and improved construction and occupies a minimum volume for a given electrical length. There is the further advantage that the invention dispenses with the need of any shield members between adjacent sections oieach folded-back conductor of the line and, thus, in a line of adjustable length avoids the use of sliding contacts or .joints necessarily employed with such members. There are the additional advantages that the characteristic impedance of the line remains substantially constant with adjustments of itselectrical length and, in a lineembodying the Fig. 4 modification of the invention, that the extent of movement required of an adjustable conductor element to effect a given adjustment of the electrical length of the line is minimized.

While there have been described what are at present considered to be the preferred embodimerits of this invention, it will. be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within he true spirit and scope of the invention.

What is claimed is:

1. A wave-signal transmission line of adjustable length comprising, an electrically contin uous conductor of U-shaped configuration having the legs thereof exposed to one another over a substantial portion of their lengths, means for adjusting the length of the legs of said conductor, means for providing on one side of said conductor a neutral surface approximately parallel to the median plane of said conductor, and conductive shield means supported in spaced relation to the other side of said conductor to pro vide on said other side a neutral surface approximately parallel to the median plane of said conductor, at least one of said neutral-surface--provioling means cooperating with said conductor to provide a folded transmission line and the spaclng between the legs of said conductor being greater than the spacing between said neutral surfaces to reduce the electric and magnetic coupling between said legs, thereby to reduce the tendency of said coupling to shorten the slicetive electrical length of said folded transmission line.

2. A wave-signal transmissionllne comprising, an electrical conductor of U-shaped configuration having the legs thereof exposed to one an other over a substantial portion of their lengths, and conductive shield means supported in spaced relation to said conductor to provide on opposite sides thereof two neutral surfaces approximately parallel to themedian plane of said conductor, said shield means co-operating with said conductor to provide a folded transmission line and the spacing between the legs ofsaid conductor being greater than the spacing between said neutral surfaces to reduce theelectric andmagnetic coupling between said legs, thereby to reduce the tendency of saidcoupling to shorten the effective electrical length of said folded transmissionline.

3. A wave-signal transmission line comprising, an electrical conductor of U-shaped configuration having the legs thereof exposed to one another over a substantial portion of their lengths, and a conductive housing enclosing said conductor in spaced relation thereto and providing on opposite sides thereof two neutral surfaces approximately parallel to the median plane of said conductor, said housing co-operating with said conductor to provide a folded transmission line and the spacing between the legs of said conductor being greater than the spacing between said neutral surfaces to reduce the electric and magnetic coupling between said legs, thereby to reduce the tendency of said coupling to shorten the effective electrical length of said folded transmission line.

4. A wave-signal transmission line of adjustable length comprising, an electrically continuous conductor of U -shaped configuration having telescoping leg sections with the legs thereof exposed to one another over a substantial portion of their lengths, means for adjusting the length of the legs of said conductor, means for providing on one side of said conductor a neutral-surface approximately parallel to the median plane of said conductor, and conductive shield means supported in spaced relation to the other s-ideo'f said conductor to provide on said other side a neutral surface approximately parallel to the median plane of said conductor, at least one of said neutral-surface-providing means co-operating with said conductor to provide a folded transmission line and the spacing between the legs of said conductor being greater than the spacing between saidneutral surfaces to reduce the electrio and magnetic coupling between said legs, thereby to reduce the tendency of said coupling to shorten the effective electrical length of said folded transmission line.

5. A wave-signal transmission line of adjustable length comprising, an electrically "continuous conductor of U sha-ped con-figuration having the legs thereof exposed to one another over a'substantial portion of their lengths, means for adjusting the length of the legs of said conductor, means for providing on one side of said conductor a neutral surface approximately parallel to the median plane of said conductor, and conductive shield means supported in spaced relation to the other side of said conductor to provide on said other side a neutral surface approximately parallel to the median plane of said conductor, at least one of said neutra'l-surface-providing means co-operating with saidconductor to provide a folded transmission line and the spacing between-the legs of said conductor being at least twice the spacing between said neutral surfaces to minimize the electric and magnetic sou-- pling between said legs, thereby to reduce the tendency of said coupling to shorten the effective electrical length of said folded transmission line.

6. A wave-signal transmission line of adjustable length comprising, an electrically continuous conductor of U-shaped configuration having the legs thereof exposed to one another over a substantial portion of their lengths, means for adjusting the length of the legs of said conductor, means for providing on one side of said conductor a neutral surface approximately parallel to the median plane of said conductor, and a conductive rectangular housing enclosing said conductor in spaced relation thereto to provide on the other side of said conductor a neutral surface approximately parallel to the median plane of said conductor, at least one of said neutralsurface means and said housing co-operating with said conductor to provide a folded transmission line and the spacing between the legs of said conductor being greater than the spacing between said neutral surfaces to reduce the electric and magnetic coupling between said legs, thereby to reduce the tendency of said coupling to shorten the effective electrical length of said folded transmission line.

7. A wave-signal transmission line of adjustable length comprising, an electrically oontinuous conductor of U-shaped configuration having the legs thereof exposed to one another over a substantial portion of their lengths, means for adjusting the length of the legs of said conductor, means for providing on one side of said conductor a neutral surface approximately parallel to the median plane of said conductor, and a conductive rectangular housing enclosing said conductor with adjacent interior wall surfaces of said housing spaced from each of said conductor legs in relation to the cross-sectional dimensions of said conductor and a value of characteristic impedance desired of said line and providing on the other side of said conductor a neutral surface approximately parallel to the median plane of said conductor, at least one of said neutral-surface means and said housing co-operating with said conductor to provide a folded transmission line and the spacing between the legs of said conductor being greater than the spacing between said neutral surfaces to reduce the electric and magnetic coupling between said legs, thereby to reduce the tendency of said coupling to shorten the effective electrical length of said folded transmission line.

8. A wave-signal transmission line comprising, an electrical conductor of U-shaped configuration having the legs thereof exposed. to one another over a substantial portion of their lengths,

for providing on oneside of said conductor a neutral surface approximately parallel to the median plane of said conductor, conductive shield means supported in spaced relation to the other side of said conductor to provide on said other side a neutral surface approximately parallel to the median plane of said conductor, at least one of said first-mentioned and secondmentioned means co-operating with said conductor to provide a folded transmission line'andthe spacing between the legs of said conductor being greater than the spacing between said neutral surfaces to reduce the electric and magnetic coupling between said legs, thereby to reduce the tendency of said coupling to shorten the effective electrical length of said folded transmission line, and means for maintaining substantially constant the characteristic impedance of said -trans-.

-'mission line 'with adjustments to its electric length.

9. A wave-signal transmission line of adjustable length comprising, an electrically continuous conductor including two linear fixed leg sections supported in parallel relation and a U-shaped section movably supported in telescopic relation with said leg sections to provide a conductor of U-shaped configuration having the legs thereof exposed to one another over a substantial portion of their lengths, means for moving said Ushaped section to adjust the length of the legs of said conductor, means for providing on one side of said conductor a neutral surface approximately parallel to the median plane of said conductor, a conductive housing enclosing said conductor and including an end wall movable with said U-shaped section of said conductor to maintain substantially constant the characteristic impedance of said line with adjustment of line length, said housing providing on the other side of said conductor a neutral surface approximately parallel to the median plane thereof, at least one of said neutral-surface means and said housing co-operating with said conductor to provide a folded transmission line and the spacing between the legs of said conductor being greater than the spacing between said neutral surfaces to reduce the electric and magnetic coupling between said legs, thereby to reduce the tendency of said coupling to shorten the effective electrical length of said folded transmission line.

10. A wave-signal transmission line of adjustable length comprising, an electrically continuous conductor of U -shaped configuration havductor a neutral surface approximately parallel to the median plane of said conductor, conductive shield means supported in spaced relation to the other side of said conductor to provide on said other side a neutral surface approximately parallel to the median plane of said conductor, at least one of said neutral-surface-providing means co-operating with said conductor to provide a folded transmission line and the spacing between the legs of said conductor being greater than the spacing between said'neutral surfaces to reduce the electric and magnetic coupling between said legs, thereby to reduce the tendency of said coupling to'shorten the effective electrical length of said folded transmission line, and dielectric material having a dielectric constant greater than unity substantially filling the space between said conductor and said neutral surfaces along at least a portion of the adjustable length of said conductor effectively to increase the electrical length of said line.

IL A wave-signaltransmission line of adjust able length "comprising, an electrically continuous conductor of L -s'haped configuration having 'the legs thereof; exposedtoone another over a substantial 'portion of their -lengths, means for adjusting the length of the legs of said conductor, means for providing on one side of said conductor a neutralsurface approximately parallel to the median plane" of said conductor, conductive shield means supported in spaced relation to the other side of said conductor to provide on said other side a'neutral surface approximately parallel to the medianplane of said conductor, at least one of said neutral surface-providing means o-operating with-said-conductor to provide a folded transmission line. and the" spacing between the legs of said conductor being greater than the spacing between said neutral surfaces to reduce the electric and magnetic coupling between said legs, thereby to reduce the tendency of said cou pling to shorten the effective electrical length of said folded transmission line, and a mixturev of a dielectric material having a dielectric constant greater than unity and finely" divided magnetic material substantially filling the space between said conductor and said neutral surfaces along at least a portion of the adjustable. length of said conductor effectively to increase thev electrical length of said line.

12. A wave-signal transmission line of, adjustable length comprising, an electrically continuous conductor including two linear leg sections of: hollow material having relatively large crosssectional area supported in, parallel relationand a U-shaped section of relatively small cross-sectional area supported in, telescopic relation with said leg sections to provide a conductor of U-shaped configuration having; the legs thereof exposed to one-another over a substantialportion of their lengths, means for providing relative movement between said leg sections and said U-shaped section toiadjust the lengthof said conductor, means for providing on one side of said conductor a neutral surface approximately parallel to the median plane. of said conductor, conductive shield means supported in spaced relation to the other side of said conductor to provide on said other side a neutral surface approximately parallel to the median: plane of said conductor, at least one of said neutral-surfacepr0- f viding means co-operating with said conductor to surfaces to reduce the electric and magnetic cou pling between said legs, thereby to reduce the tendency of said coupling to shorten the effective electrical length of said: folded transmission line, and dielectric material having a dielectric constant greater than unity substantially filling the space between said U-shapedsection of saidconductor and said neutral surfaces-effectively to increase-the electrical length of said line.

13. A wave-signal transmission line of adjustable length comprising, an electrically continuous conductor including two linear leg sections of hollow material having relatively large crosssectional area supported in parallel relation and a U-shaped section of relatively small cross-sectional area supported in telescopic relation with said leg: sections to provide a conductor of U-shaped configuration having the legs thereof exposed to one another over a substantial portion of their lengths, means for providing relative movement between said leg sections and said U-shaped section to adjust the length of said conductor, means for providing on one side of said conductor a neutral surface. approximately parallel, to the median plane of said conductor,

conductive shield means supportedinvspaced relation to the other side of said conductor to provide on said other side a neutral. surface approximately parallel to the median plane of said conductor, at least one of said neutral-surfaceproviding means co-operating with said.v conductor to provide a folded transmission; line and the spacing between the legs of said conductor being greater than the spacing between said neutral surfaces to reduce the electric and magnetic coupling between said legs, thereby to reduce the tendency of said coupling to shorten the effective electrical length of said folded transmission line, and a mixture of adielectric, material having a dielectric constant greater than unity and finely divided magnetic. material substantially filling the space between said U-shaped section of said conductor and said neutral surfaces, effectively to increase the electrical length of said line.

14. A balanced wave-signal transmission line comprising, a pair of conductors each of U-shaped configuration supported in approximately parallel spaced relation to provide a folded transmission line, each of said conductors having the legs thereof exposed to one another over a substantial portion of their lengths and providing for the other conductor in a region, intermediate said conductors a neutral surface approximately parallel to the median planes of said conductors, and conductive shield means supported in spaced relation to said conductors and on opposite sides of said pair of conductors to provide on, said 019,-- positesides neutral surfaces approximately parallel to the median planes of said conductors, the spacing between the legs of each of said conductors being greater than the spacing between the neutral surfaces provided on opposite sides of said each conductor to reduce the electric and magnetic coupling between the legs of said each conductor, thereby to reducethe tendency of said coupling, to shorten the: effective electrical length of said folded transmission line,

15. A balanced wave-signal transmission line of adjustable length comprising, a pair of electrically continuous conductors each of U-shaped configuration supported in approximately parallel spaced relation to provide a folded transmission line, means for simultaneously adjusting the lengths of the legs of said conductors, each of said conductors having the legs thereof exposed to one another over a substantial portion of their lengths. and providing for the other conductor in a region intermediate said, conductors a neutral surface approximately parallel to the median planes of said conductors, and. conductive shield means supported in individual spaced relation to said conductors and on opposite sides of said pair of conductors to provide on said opposite sides neutral surfaces approximately parallel to the median planes of said conductors, the spacing between the legs of each of said conductors being greater than the spacing between the neutral surfaces provided on opposite. sides of said each conductor to reduce the electric and magnetic coupling between the legs of said each conductor, thereby to reduce the tendency of said coupling to shorten the effective electrical length of said folded transmission line.

HAROLD A. WHEELER.

REFERENCES CITED The following reference" are of record in the file of this patent:

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